1. Field of the Invention
The invention relates in general to a read only memory (ROM) diode and a method of making the same. More particularly, the present invention relates to a ROM that uses a diode as a memory cell, and a method of making such a memory cell.
2. Description of the Related Art
Read only memory is widely used in mini computers, micro processors and other digital hardware. A ROM can be used to store system information, for example, basic input/output system (BIOS) information. However, ROM devices are difficult to manufacture and the various manufacturing and pretreatment steps are time consuming. Therefore, customers typically deliver programming data to the factory that makes the ROM devices, which then programs the ROM device to complete the product.
Because ROM devices typically have similar structures (with the exception of the data which is stored in the ROM device during the programming step), in most integrated circuit (IC) factories a ROM is produced without performing the programming steps, and is stored. After receiving the program from the customer, a mask is produced so that the necessary programming step can be performed and production finished, thus reducing the lead time and thereby providing better customer service. Therefore, a ROM that is post-programmed using a mask is commonly used in the art.
Channel transistors are normally used as memory cells for ROM. In the programming process, specific channel regions of the channel transistors are selectively implanted with impurities, thus modifying the threshold voltages of the transistors. This controls the conductivity of the memory cell. A detailed description of a conventional ROM structure is as follows, with reference to FIG. 1A through FIG. 1C. FIG. 1A is a top, partial view of a conventional ROM. FIG. 1B is a front, partial view of the conventional ROM. FIG. 1C is a cross-sectional view of the ROM. The conventional ROM includes a substrate 10, a plurality of bit lines 11 (BL), an oxide layer 12, and a plurality of word lines 13. The substrate 10 is, for example, a P-type substrate. The bit lines 11, oxide layer 12, and word lines 13 are formed on the substrate 10. As shown in FIG. 1A, the region surrounded by the dotted line forms a memory cell 14. Data in binary form, such as "0" or "1", is stored in the memory cell by implantation of a channel 16 (FIG. 1c).
Referring also to FIG. 1C, an N-type implantation is performed using, for example, arsenic ions to form bit lines 11, with the bit lines 11 being arranged with an equal distance between each other. Channel 16 is formed between adjacent bit lines 11. Next, oxide layer 12 is formed on the surface of bit lines 11 and channel 16 using an oxidation process. A conductive layer, for example, a layer of heavy doped polysilicon, is formed over the substrate 10. Using photolithography and etching processes, word lines 13 (which cross bit lines 11) are formed from the conductive layer. Next, channel transistors are formed to complete the manufacture of a conventional ROM (with the exception of programming processing). Next, a series of continuous programming steps is performed. A mask 15 is formed over the ROM to expose the portions of channel 16 which are to be encoded. Then, a P-type implantation is performed using, for example, boron ions to implant a code. However, the type of the implanted impurity is selected depending on the characteristics of the transistor.
Channel transistors are typically used as memory cells in the conventional ROM described above. As is well known, the process of making the channel transistor is complex. Because a metal-oxide-semiconductor transistor is difficult to minimize, the integration of devices can not be effectively performed. Further, in the manufacturing of the conventional ROM, the bit lines are formed using N-type implantation. Because the resistance of the bit lines is about 100 ohm per square, operating current cannot be increased. Further, because of the lower breakdown voltage of the channel transistor, the decoding operating voltage is limited. These limitations may result in an output error.